How the associative strengths of stimuli combine in compound: summation and overshadowing

When two conditioned stimuli (CSs) are presented in compound, the response is typically stronger than to the individual CSs, implying that their associative strengths combine. However, to identify exactly how associative strengths combine requires an accurate description of the relationship between associative strength and responding. The authors have used the delta rule (Rescorla & Wagner, 1972) to constrain the predicted growth of associative strength (V) to identify the relationship between V and responding across the course of Pavlovian conditioning of two CSs (one auditory, one visual). Responding to the compound was best predicted as 0.6xV(CS1)+0.6xV(CS2), suggesting that only 60% of the associative strength of each CS generalized to the compound. A second experiment confirmed this result and additionally showed that summation of responding between two same-modality CSs (both auditory or both visual) declined across training. A third experiment applied the procedure to compound conditioning, showing that responding to the compound was equal to the sum of the response rates to the individual CSs. The results are discussed in terms of configural and elemental models of Pavlovian conditioning.